Recent observations have shown the presence of thyrotropin-releasing hormone-like material (TRH-LM) in rat pancreatic islets and in retina. Its immunological and biological properties are identical to those of synthetic thyrotropin-releasing hormone (thyroliberin). This communication deals with the ontogenesis of TRH-LM in rat pancreas and retina as compared to that of rat hypothalamus. Effects of sex and exposure to constant dark were also studied. Results show that asynchronous changes in the concentration of TRH-LM occur during the postnatal maturation of these tissues, presumably mediated by organ-specific control mechanisms--e.g., light affects only the accumulation of TRH-LM in the retina. TRH-LM may act as neurotransmitter in the regulation of pancreatic islet cell function and in the development of photo-reception in the retina. Increases in hypothalamic TRH-LM seem to parallel the development of the pituitary-thyroid secretory activity, but the function of extrahypothalamic TRH-LM remains speculative.